Update TestValidation/Timestamp_Validation_test_4

[polds]

TestValidation/Timestamp_Validation_test_4 is broken. We should fix or remove.

=== RUN   TestValidation/Timestamp_Validation_test_4
    eval_test.go:349: Compile() got error = unknown name timestamp (1:1)
         | timestamp(-6213559680)
         | ^, wantErr false
    --- FAIL: TestValidation/Timestamp_Validation_test_4 (0.00s)